Folding knives are popular due to their compact size and portability. However, a potential drawback of certain folding knives is that they require two hands for use. One hand is required for holding the handle portion of the blade, while the other hand is necessary to withdraw the blade from its retracted position within the handle and move the blade to the extended, operable position.
In most assisted-opening folding knives, a leaf torsion spring or straight wire spring is used, so that when the knife is opened, the spring provides an initial assist to move the blade, but the continued opening of the blade relies on inertia to complete the opening process.
For example, the folding knife disclosed in U.S. Pat. No. 5,815,927 (Collins) allows the user to extract the blade from its refracted position within the handle by the user's engaging and pulling on ridges defined in a rear portion of the blade. A plunger mechanism having a coil spring facilitates opening of the blade, but the user is still required to pull on the ridges in order to move the blade through a selected range of motion and for the spring to assist the blade to move it to the extended position.
Many folding knives have been patented. U.S. Pat. No. 273,858 (Korn) discloses a folding knife having a leaf-type spring for moving a blade to an extended position. U.S. Pat. No. 1,603,914 (Hermann) discloses a folding knife having a coil spring connected to a metal tape, which pulls the blade to a retracted position. U.S. Pat. No. 2,601,999 (Sly) discloses a foldable gaff hook having a similar opening mechanism. U.S. Pat. No. 2,407,897 (Newman) discloses a spring for pivoting blade open upon actuation of a locking lever. U.S. Pat. No. 698,080 (Treas) discloses use of an actuating spring for pivoting a blade to an open position. U.S. Pat. No. 4,535,539 (Friedman, et al.) and U.S. Pat. No. 5,093,995 (Jan) disclose button release mechanisms for folding knives. U.S. Pat. No. 4,893,409 (Poehlmann) and U.S. Pat. No. 5,964,035 (Poehlmann) disclose folding knives having adjustment screws for adjusting the fit of the blade in the extended position. U.S. Pat. No. 1,065,863 (Carter) also discloses use of set, or an adjustment, screw. U.S. Pat. No. 6,397,477 (Collins) discloses a spring-assisted folding knife which initiates pivoting a blade from a retracted position to a locked extended position. These knives typically use either inertia, gravity-assist, or a flick of the wrist to complete the opening process.